Polycarbonates in general are derived from the random, statistical reaction of one or more dihydric phenols and a carbonate precursor, particularly phosgene. Such polycarbonates are high temperature, high performance thermoplastic polymers with a good combination of thermal and mechanical properties. Polycarbonates also have good processability which allows them to be molded into a variety of articles.
However, when such polycarbonates are exposed to a hydrolytic environment under excessive time and/or temperature conditions, the hydrolytic stability of the polycarbonates is generally poor which results in poor mechanical properties. Poor hydrolytic stability is reflected by a rapid decrease in reduced viscosity of the polycarbonates resulting from exposure to the hydrolytic environment. This deficiency also requires careful drying procedures of the polycarbonates prior to melt processing.
Bis-(3,5-dimethyl-4-hydroxyphenyl)sulfone (TMBS) and 2,2-bis-(4-hydroxyphenyl) propane (Bisphenol A) have been variously described for use in making polycarbonate resins. U.S. Pat. No. 3,737,409 describes a random, non-ordered copolymer of the reaction product of TMBS, Bisphenol A and a carbonate precursor such as phosgene wherein the bisphenol reaction mixture of TMBS and Bisphenol A is comprised of 40-99 weight percent of TMBS and a corresponding 60-1 weight percent of Bisphenol A. The claims of the patent are directed to a copolymer as above described except that amount of TMBS used in the copolymer is 50-99 weight percent (and 50- 1 weight percent of Bisphenol A based on the total weight of TMBS and Bisphenol A). Such proportions of these monomers in the manufacture of the polymer are urged by the patentee to enhance the hydrolytic stability of the resultant polycarbonate.
European Patent Publication No. 0,000,547 published July 2, 1979, discloses that random, non-ordered copolycarbonates of an aromatic diphenol (e.g. Bisphenol A), an aromatic sulfonyl diphenol (e.g. bis-(alkylsubstituted-4-hydroxyphenyl) sulfone) and a carbonate precursor such as phosgene exhibit improved heat deflection temperatures when the amount of sulfonyl diphenol is from 1% to 50% by weight of total diphenol and said sulfonyl diphenol is at least 99% pure 4--4' isomer by weight of total sulfonyl diphenol.
There is described by Serini in Angewandte Makrom. Chemie. 55 (1976) pages 175-189 (Number 855) the manufacture of high molecular weight random, non-ordered polycarbonates by interfacial polycondensation of o, o, o', o'-tetramethylsubstituted bisphenols and phosgene. According to the reference, the synthesis of these polycarbonates requires the use of high concentrations of catalyst, high pH values, excess amounts of phosgene and a long condensation period. The polycarbonates are characterized by high second order transition temperatures, excellent hydrolytic stability, lower refractive indices and good thermoplastic processability. However, at page 182, the authors state that a polycarbonate based on bis-(3,5-dimethyl-4-hydroxyphenyl)sulfone is not melt stable.
Copending U.S. patent application Ser. No. 485,945, filed Apr. 18, 1983, is primarily directed to improved random, non-ordered polycarbonates comprising from about 20 weight percent to about 95 weight percent of repeating units (I) having the formula ##STR1## and from about 5 weight percent to about 80 weight percent of repeating units (II) having the formula ##STR2## in which the repeating units (I) and the repeating units (II) are connected by interbonding units (III) having the formula ##STR3## wherein Y is selected from alkyl groups of 1 to 4 carbon atoms, chlorine or bromine, each z, independently, has a value of from 0 to 4 inclusive, n has a value of 0 or 1, and R.sub.1 is a divalent saturated or unsaturated aliphatic hydrocarbon radical, O, CO, SO.sub.2, S or a direct bond, with the proviso that when R.sub.1 is SO.sub.2 then repeating unit (II) is not the same as repeating unit (I) and wherein terminal units in the polycarbonate are bonded to repeating units (II). The invention is aIso directed to a phosgenation process for preparing the improved polycarbonates.
In general, polycarbonates prepared by phosgenation processes exhibit a random, statistical arrangement of repeating units in the polymer chain rather than an ordered, nonstatistical arrangement. It has been found as a result of this invention that polycarbonates which contain ordered, alternating carbonate units derived from bis-(3,5-dimethyl-4-hydroxyphenyl)sulfone (TMBS) and a bisphenol compound such as 2,2-bis-(4-hydroxyphenyl)propane (Bisphenol A) exhibit improved hydrolytic stability over that shown by the above cited prior art. Such improved hydrolytic stability is attributable to the ordered arrangement of repeating units derived from TMBS in the polymer chain and in particular, to the molecular configuration of repeating units derived from TMBS in which the four methyl groups attached to the aromatic rings of TMBS act to sterically hinder hydrolytic attack against the carbonyl-ether oxygen linkages vicinally positioned in the polymer chain.
In addition to improved hydrolytic stability, the polycarbonates of this invention having ordered, alternating carbonate units derived from TMBS and a bisphenol compound such as Bisphenol A exhibit excellent processability in molding applications. The polycarbonates of this invention therefore exhibit a highly desirable combination of properties not afforded by polycarbonates having a random, statistical arrangement of similar carbonate repeating units prepared by phosgenation processes known in the art.